1. Field of the Invention
The invention relates to a seed used for crystalline silicon ingot casting, and particularly to a seed capable of reducing red zone and yellow zone of a crystalline silicon ingot fabricated by such seed.
2. Description of the Prior Art
Most of the photovoltaic cells produce a photovoltaic effect when absorbing sunlight. Currently, the photovoltaic cell is made of a silicon-based material, since for the most parts; silicon is the second most abundant and accessible element in the world. Also, silicon is cost-effective, nontoxic, and chemically stable, and becomes broadly used in semiconductor applications.
There are three forms of crystalline silicon for fabricating silicon-based photovoltaic cells, i.e., mono-crystalline silicon, multi-crystalline or polycrystalline silicon, and amorphous silicon. Multi-crystalline or polycrystalline silicon is much less expensive than mono-crystalline silicon when produced by Czochralski (CZ) method or float zone (FZ) method, so it is usually used as a raw material of the photovoltaic cell due to the economic concern.
Conventionally, multi-crystalline or polycrystalline silicon for photovoltaic cells is fabricated by a common casting process. That is, it is prior art to produce multi-crystalline or polycrystalline silicon for photovoltaic cells by a casting process. In brief, the multi-crystalline or polycrystalline silicon photovoltaic cell is fabricated by melting high purity silicon in a mold like quartz crucible, then cooling the melted silicon in a controlled solidification to form a multi-crystalline or polycrystalline silicon ingot. The multi-crystalline or polycrystalline silicon ingot is generally cut into bricks having a cross-section that is the same as or close to the size of the wafer to be used for manufacturing a photovoltaic cell, and the bricks are sawed or otherwise cut into such wafers. The ploy-Si produced in such manner is an agglomeration of crystal grains where, within the wafers made therefrom, the orientation of the grains relative to one another is effectively random.
The random orientation of grains, in either conventional multi-crystalline or poly-crystalline silicon, makes it difficult to texture the surface of a resulting wafer. Texturing is used to improve efficiency of a photovoltaic cell, by reducing light reflection and improving light energy absorption through the surface a cell. Additionally, “kinks” that form in the boundaries between the grains of conventional multi-crystalline or poly-crystalline silicon tend to nucleate structural defects in the form of clusters or lines of dislocations. These dislocations, and the impurities they tend to attract, are believed to cause a fast recombination of electrical charge carriers in a functioning photovoltaic cell made from conventional multi-crystalline or poly-crystalline silicon. This can cause a decrease in the efficiency of the cell. Photovoltaic cells made from such multi-crystalline or poly-crystalline silicon generally have lower efficiency compared to equivalent photovoltaic cells made from monocrystalline silicon, even considering the radial distribution of defects present in monocrystalline silicon produced by known techniques. However, because of the relative simplicity and lower costs for manufacturing conventional multi-crystalline or poly-crystalline silicon, as well as effective defect passivation in cell processing, multi-crystalline or poly-crystalline silicon is a more widely used form of silicon for manufacturing photovoltaic cells.
Currently, it has been developed that crystalline silicon ingot is fabricated using a mono-crystalline silicon seed layer and based on directional solidification. In this way, a high quality ingot of mono-crystalline silicon and/or bi-crystal silicon block or mono-like crystal silicon block may be obtained, in which the lifetime of the minority carriers is maximized in the resultant wafer employed for fabricating a high-performance photovoltaic cell. As used herein, the term “mono-crystalline silicon” refers to a body of single crystal silicon, having one consistent crystal orientation throughout. The term “bi-crystal silicon” refers to a body of silicon, having one consistent crystal orientation throughout for greater than or equal to 50% by volume of the body, and another consistent crystal orientation for the remainder of the volume of the body. For example, such bi-crystal silicon may include a body of single crystal silicon having one crystal orientation next to another body of single crystal silicon having a different crystal orientation making up the balance of the volume of crystalline silicon. The term “mono-like crystal silicon” refers to a body of silicon, having one consistent crystal orientation throughout for greater than 75% by volume of the body. Additionally, conventional multi-crystalline silicon refers to crystalline silicon having cm-scale grain size distribution, with multiple randomly oriented crystals located within a body of silicon. The term “poly-crystalline silicon” refers to crystalline silicon with micron order grain size and multiple grain orientations located within a given body of silicon. For example, the grains are typically an average of about submicron to sub-millimeter in size (e.g., individual grains may not be visible to the naked eye), and grain orientation distributed randomly throughout.
It has also been developed that crystalline silicon ingot is fabricated using a nucleation promotion layer constituted by granulars of mono-crystalline or poly-crystalline silicon to assist in nucleation of silicon grains and based on direction solidification. The resultant crystalline silicon ingot has small-sizes silicon grains at the bottom thereof and low density of bulk defects, and can be used for fabricating a high-performance photovoltaic cell.
The region in crystalline silicon ingot generally fabricated using crucible and not meeting requirement represents as “red zone”. The photovoltaic cell made from the red zone of general crystalline silicon ingot has low minority carrier lifetime. Red zone in the minority carrier lifetime mapping, obtained by a measurement way such as a microwave photoconductive decay (μ-PCD) way, shows red image. The causes of red zone include: a. the region containing impurities in solid state diffusion from crucible; b. the region of non-perfect crystal structure near the crucible; c. the boron-rich or oxygen-rich region; and d. the crystalline seeds and nucleation layer containing metal in liquid state diffusion from the silicon melt containing metal. In general, the region containing impurities, especially metal impurities, in solid state diffusion from crucible is the primary cause of red zone. The photoelectric conversion efficiency of photovoltaic cell made from the red zone of crystalline silicon ingot severely decays and decreases.
Yellow image in the minority carrier lifetime mapping, obtained by a measurement way such as a μ-PCD way, represents as “yellow zone” which is also region polluted by impurities. The causes of yellow zone is that metal impurities in the crystalline seeds polluted by silicon melt diffuse back into the bottom of crystalline silicon ingot during initial seeding of the crystalline silicon ingot, where diffusion paths of metal impurities include grain boundary diffusion and solid state diffusion. Thus, some portion of yellow zone shows filamentous pattern. Yellow zone is less available region of the crystalline silicon ingot, but the photoelectric conversion efficiency of photovoltaic cell made from the yellow zone of crystalline silicon ingot is lower.
However, red zone of current crystalline silicon ingots, fabricated using a layer of single crystal seeds or a nucleation promotion layer of mono-crystalline or poly-crystalline silicon granulars disposed at the bottom of crucible, is larger than or even twice as much as that of crystalline silicon ingots fabricated using no above layers. Yellow zone of the crystalline silicon ingots fabricated using above layers is also lager than that of the crystalline silicon ingots fabricated using no above layers. Studying its causes, during fabrication of the crystalline silicon ingot, impurities primarily consisting of metal impurities (e.g., Fe, Al, etc.) in the crucible are dissolved in the silicon melt, and then diffuse into single crystal seeds or mono-crystalline or poly-crystalline silicon granulars. As silicon grains from the silicon melt nucleate and grow at the single crystal seeds or the mono-crystalline or poly-crystalline silicon granulars, the impurities in the single crystal seeds or the mono-crystalline or poly-crystalline silicon granulars will diffuse back into solidified silicon crystals.